Abstract

The in-plane fourfold symmetry of the upper critical fieldHc2 in a stripe-ordered La1.45Nd0.4Sr0.15CuO4single crystal has been observed through measuring the out-of-plane resistivityρc by rotating the crystal around its c-axis in magnetic fields parallel to the ab-plane of the crystal. It was found that the upper critical fieldHc2 takes maxima (minima) for magnetic fields along Cu-O-Cu (Cu-Cu) direction, suggesting an intrinsic fourfold superconductivity. The results should be closely related to the special structure of the charge stripes which are along Cu-O-Cu direction and rotated 90° in adjacent CuO2 planes, and the fourfold symmetry behaviors in Hc2 and ρc can be well described by a scaling method. The stripe phase can also lead to a fourfold vortex pinning effect which is confirmed by the angular dependence of the irreversibility field Hirr and the activation energy U for flux motion.